1. Field of the Invention
The present invention relates to an optical device used in an optical pickup for recording, reproducing, or erasing information in an optical recording medium, a method of producing the optical device, an optical product, an optical pickup and an optical information processing device.
2. Description of the Related Art
In the related art, an optical pickup in the present technical field is constituted by assembling many optical parts such as lenses, prisms, wave plates, polarized-light optical devices, and so on. In recent years and continuing, it is required that an optical pickup be able to support not only conventional CDs or DVDs, but also new optical recording media conforming with plural new and old standards, such as large capacity blue-light optical recording media. To meet this requirement, the number of the optical parts may further increase. On the other hand, the size of the optical pick is limited, and cannot be made larger than this limit. In other words, it is necessary to include DVD, or blue light functions into a space having a CD size. Furthermore, it is required to make the optical pickup more compact, when, for example, the optical pickup is installed in a notebook personal computer.
As for the optical parts, such as a prism of a polarized beam splitter, there exist problems of many processing steps and a large space occupied in the arrangement. Specifically, the prism of the polarized beam splitter includes two right-angle glass prisms whose shapes are processed at high precision, and after a wavelength selection film is formed on the slope of one prism, the one prism is combined with the slope of the other prism, forming a cubic structure. Due to this structure, complicated processing steps are required compared to plate elements. In addition, an optical part having such a cubic structure occupies considerable space inside the optical pickup.
As for materials of optical parts, in the related art, a ¼ wave plate is made from a quartz crystal, which is an optical crystal, or from a liquid crystal. However, when using the quartz crystal, because it is necessary to process the optical crystal with the primary axis of the crystal being in a specified direction, and direction precision is also required when assembling the optical pickup, there is a negative effect on cost reduction. When using the liquid crystal, the liquid crystal has to be sealed by using two glass substrates, and this also causes increased cost.
Therefore, it is desired that by integrating plural optical parts into an optical device, an optical device can be made compact and be assembled simply, and the cost of the optical device can be further reduced without using an evaporation film or an optical crystal, which are fabricated in each conventional optical part.
Along with recent progress in processing techniques, it becomes possible to fabricate a grating structure having a pitch comparable with the light wavelength or even shorter. In such a sub-wavelength grating structure, although a diffracted wave is not generated, the transmittance properties strongly depend on the fine structure, and it is possible to control a phase speed (effective refractive index), or properties of polarized light by controlling the fine structure. For example, this technique is described in the following references.    1. Hisao Kikuta, Koichi Iwata, “Structural complex refractive index and its applications to optical devices”, in “Introduction to Diffraction Optical Devices”, under the editorship of Physical Society of Applied Physics, Optical Society of Japan, Optical Design Group, Optronics Co., May 20, 1997, first edition, first copy, pp 158.    2. Hisao Kikuta, Koichi Iwata, “Optical control with a fine grating structure comparable to light wavelength”, Optics, Vol 27, pp. 12-17 (1998).    3. Hisao Kikuta, “Diffraction grating in sub-wavelength region”, Oplus E, Vol. 21, No. 5 (May, 1999) pp. 543-550.    4. Japanese Patent Gazette No. 3077156.    5. Japanese Patent Gazette No. 3382600.
By using the grating structure in the sub-wavelength region, for example, it is not necessary to perform surface coating of prisms of a polarized beam splitter, and the structure can be made into a plate. Similarly, because the phase speed can be controlled, it is possible to produce a ¼ wave plate by a grating structure.
FIG. 24 is a schematic view of a polarized light selective diffraction device using the sub-wavelength grating in the related art.
In FIG. 24, rectangular gratings having fine pitches are arranged periodically at intervals longer than the wavelength of incident light. If a phase difference between the light passing through the rectangular gratings of fine pitches and the light passing through regions other than the rectangular gratings is an integral multiple of 2π, the incident light is not diffracted and the incident light beam is totally transmitted through the diffraction device. While, if the phase difference is an integral multiple of π, all of the incident light is diffracted, and there is no light directly passing through the diffraction device. By appropriately selecting the equivalent refractive indexes and heights of the rectangular gratings of fine pitches, it is possible to separate the incident polarized light with light diffraction. In addition, diffraction direction can be controlled by the shape of the grating of pitches longer than the light wavelength.
FIG. 25 is a schematic view of a wave plate using the sub-wavelength grating in the related art.
In FIG. 25, a phase difference is obtainable from anisotropy generated by the sub-wavelength grating, and since this phase difference can be set to be π or π/2, various kinds of wave plates can be realized.
Further, while the sub-wavelength grating is formed in the whole region of the optical device in the related art, in an optical pickup of the present invention, the sub-wavelength grating is formed only in a specified limited region to constitute various kinds of optical elements.
However, when the sub-wavelength grating is formed only in a limited region, a light path length difference, and in turn, a phase difference is generated between the region where the sub-wavelength grating is formed and the region where the sub-wavelength grating is not formed; due to this, a wave front aberration is generated, and this degrades the light condensing properties of the object lens.
FIG. 26A is schematic view illustrating light paths in the sub-wavelength grating and in the region without the sub-wavelength grating in the related art.
As shown in FIG. 26A, the sub-wavelength grating is formed only in a specified region. If the heights of the sub-wavelength grating and the region without the sub-wavelength grating are not appropriately selected, the light path in the sub-wavelength grating is different from the light path in the region without the sub-wavelength grating.
The light path difference Δnd can be expressed byΔnd=(1+ns)*d/2−n1*d 
where ns represents a refractive index in a vertical polarization direction, and n1 represents a refractive index of the sub-wavelength grating region.
This light path difference Δnd generates a phase difference between the sub-wavelength grating region and the region without the sub-wavelength grating.
FIG. 26B illustrates an aberration caused by the phase difference in the related art.
As illustrated in FIG. 26B, due to the light path difference between the sub-wavelength grating region and the region without the sub-wavelength grating, and in turn the phase difference between the sub-wavelength grating region and the region without the sub-wavelength grating, a wave front aberration is generated, and this degrades the light condensing properties of the object lens.